Over the last two decades, renewed interest and expertise in and about breastfeeding has occurred in the U.S. and Western Europe. Human milk is universally accepted as the ideal food for infants and breastfeeding has real and potential benefits for the mother, the infant and to their relationship. In the United States only 50% of women choose to breastfeed their infants despite all of the real and potential advantages to both the infant and mother and less than half of these are successful. The primary determinant of milk volume is lactose production. If we understood the carbon source for lactose synthesis and the hormonal and substrate factors which regulate lactose production, we could design new strategies to improve the number of women who can breastfeed successfully. The present proposal is designed to determine: a. the metabolic consequences of breastfeeding in women, b. metabolic precursors of lactose and c. the hormone and substrate factors regulating lactose synthesis. The overriding hypotheses of this proposal are: 1. Regulation of lactose synthesis controls the production of milk in humans. 2. Understanding the metabolic adaptation to lactation and processes that control milk production will result in improved success in lactation and appropriate methods to sustain weight loss while maintaining appropriate milk production in lactating women who have gained excessive weight during pregnancy. We have provided preliminary data that rhGH increases milk production at least in part by increasing the transcription of alpha-Iactalbumin. In the protocols described below, we will test the following specific hypotheses: 1. Lactating women have increased insulin sensitivity and increased glucose effectiveness when compared to that of nonlactating women. 2. Lactate is the primary substrate for hexoneogenesis in the breast. 3. Glutamine and alanine both contribute to hexoneogenesis in the breast. 4. rhGH increases the rates of synthesis of alpha-Iactalbumin mRNA and GLUT 1 and GLUT-3 transporters mRNAs but not those of PEPCK, key gluconeogenic enzymes. 5. RhGH will increase lactose (milk) production in mothers delivering premature infants and increase the long-term success rate in breast-feeding. 6. When compared to an isonitrogenous, isocaloric low carbohydrate/high fat diet, a high carbohydrate/low fat diet over a 7 day period: a. Milk production will be greater on the high carbohydrate, and b. Rates of gluconeogenesis (and hexoneogenesis) will be lower during both the overnight fasting and the fed states. Insulin sensitivity will be assessed by use of the stable labeled ivgtt. Infusions of [13C]labeled alanine, glutamine or lactate will be used to determine their relative contribution to the de novo synthesis (hexoneogenesis) of the glucose and galactose in breast milk lactose; The effects of both acute and chronic rhGH administration will be assessed looking at milk volume and the induction of specific mammary gland mRNA (alpha-Iactalbumin, GLUT 1, GLUT 3, and PEPCK) in lactating women with established breast feeding with a healthy term infant and in mothers of premature infants using prospective blinded trial to determine its effect on milk production, mRNA expression and long-term success in breastfeeding. Finally, dietary manipulations will be utilized to determine if altering the dietary macronutrient distribution (55% fat/25% carbohydrate vs. 25% fat/60% carbohydrate) of a lactating woman will adversely affect milk production, hexoneogenesis of lactose, or milk composition. Understanding the metabolic consequences of lactation on maternal metabolism and the regulation of lactose synthesis will give new insights into the regulation of human milk production, providing new strategies to improve the number of successfully breastfeeding women and to allow women to decrease unwanted weight while maintaining adequate milk production.